The present invention relates to a time axis control method which is utilized during recording or playback of data such as video signals or audio signals on or from a medium such as a disk or magnetic tape.
FIG. 1 shows an example of a method of time axis control for a video disk player. Reference numeral 1 denotes a video disk, and reference numeral 2 a spindle motor which rotates the video disk 1. A signal which is produced by a signal recording/playback unit including a pick-up (not shown in the drawings) is demodulated by a demodulator circuit 3, and the demodulated signal thus produced is transferred through a CCD (charge coupled device) 4 which serves as a fine time axis adjustment unit. Part of the resultant output signal from the CCD 4 is supplied to a circuit 5 which performs sync separation and removal of equalizing pulses to thereby produce pulses which are synchronized with the playback horizontal sync signal. These pulses are input to a phase comparator 6 and are compared in phase therein with an input signal from a reference frequency oscillator 7. The output signal from this oscillator has the same frequency as the playback horizontal sync signal (i.e., 15.734 KHz in the case of the NTSC system and 15.625 KHz in the case of the PAL system). The phase error output signal which is output from the phase comparator 6 is transferred through an amplifier 8, equalizer 9, amplifier 10, equalizer 11 and amplifier 12 to the motor 2. In addition, the output signal from the equalizer 9 is applied to a VCO (voltage-controlled oscillator) 13 to control the clock pulses which are appled to the CCD. In this way, coarse time axis control is performed by a loop including the demodulator circuit 3, CCD 4, sync separator circuit 5, phase comparator 6, amplifier 8, equalizer 9, amplifier 10, equalizer 11, amplifier 12 and motor 2. On the other hand, fine time axis control is performed by a loop comprising sync separator circuit 5, phase comparator 6, amplifier 8, equalizer 9, VCO 13 and CCD 4.
During a skip-scanning operation, in which the pick-up is alternately and successively set in a track jumping mode of operation whereby one or more tracks are skipped over, and a tracking servo mode of operation, in which the pick-up is made to follow along a track while time axis control is continued, playback of the video signal will take place during each of the intervals in which tracking occurs. Corresponding images will appear upon the monitor. Thus, such a skip-scan operation is analogous to looking up pages in a book.
In the case of a type of CAV (constant angular velocity) disk in which the positions at which the horizontal and vertical sync signals are recorded are positioned along radial lines on the disk, no problems will arise with such scanning. However, in the case of a CLV (constant linear velocity) or a type of CAV (constant angular velocity) disk in which the positions at which the horizontal and vertical sync signals are recorded do not lie along the disk radius, discontinuities will arise between the signal phase prior to a track jumping operation and the phase subsequent to that operation. Thus, each time a track jump operation takes place, a large-amplitude error signal will be produced by the time axis servo system of the disk player, and hence a relatively long time will be required before stable operation is restored. In addition, there is a danger that erroneous operation may result due to the maximum compensation range of the CCD being exceeded. For this reason, when a comparatively large-amplitude time axis error is produced, the fine time axis adjustment unit constituted by the CCD loop is placed in the off state, and only the coarse time axis adjustment unit (i.e., the spindle motor servo loop) is set in operation. As a result, color sync is lost from the displayed monitor picture, causing a black-and-white image to be displayed. A further disadvantage of this method is that, when the scanning operation is terminated and normal playing is restarted, a relatively long time must elapse before time axis control can be stabilized, that is, to reduce the time axis error to a sufficiently low value. During that time, an objectionable coloration state will appear on the monitor display.
A proposed method of overcoming the problems described above is disclosed in Japanese Pat. No. 58-98881. With that method, during a track jump operation, a hold condition is established for a frequency divider used as a counter. The hold state is implemented by a gate pulse which occurs during the interval in which a track jump operation takes place. In this way, it is proposed to ensure smooth continuity for the time axis error without disturbances occurring in the value of the error.
However, in fact, even when the technique described in the above-mentioned patent is employed, color disturbances still appear on the displayed monitor image due to disturbances of the time axis data while scanning operations are repetitively performed. This is due to the fact that, during the scanning operation, when the limit of the range of movement of the tracking mirror (which forms part of the signal recording/playback unit) is approached, a "tracking servo loop open" pulse is generated, which causes the mirror to be restored to a position close to the center of its range of movement. These "tracking servo loop open" pulses are generated in a manner which is unrelated to the period of the time axis error. Thus, when scanning operations are repetitively performed, problems will arise.
Firstly, although the time axis error is applied in a smoothly continuous fashion, the process of correction of the time axis error (commencing after each track jump operation when the CCD is reconnected) will begin from the condition existing immediately prior to that track jump, i.e., from a condition in which a certain amount of offset is contained in the error quantity.
Another problem is that the distance between the center position of a track jump and the positions at which the jump begins and ends (measured radially along the disk surface) will correspond to a substantial number of tracks. Thus, in the case of a CLV disk, the speed of rotation of the spindle motor should change by an amount which corresponds to that radial position difference. However due to the slow speed of response of the spindle motor, it is necessary for this compensation to be performed by the CCD, which has a high speed of response. When such track jump operations are performed repetitively, the offset value (the operating point deviation) will tend to accumulate and gradually increase. If this can be compensated by appropriate rotation of the spindle motor, then no problem will arise. However, if a large amount of offset is generated within a short time interval, then the spindle motor will not be able to respond with sufficient rapidity, and eventually the limit of compensation by the CCD will be reached. Loss of display color and time axis disturbances will then result.